It is known from the prior art described in U.S. Pat. No. 4,153,205 and in U.S. Pat. No. 3,980,237 to form the valve member so that the seating line is defined by the meeting line of two conical surfaces the upstream one of which has a cone angle which is less than the cone angle of the frusto conical seating and the downstream one of which has a cone angle which is greater than that of the seating. In one example the included angle between the conical surface of the seating and the downstream conical surface of the valve member is greater than the included angle between the conical surface of the seating and the upstream conical surface of the valve member. With this arrangement as the surfaces wear the effective seating line moves in the upstream direction so that the nozzle opening pressure increases assuming that there is no substantial deterioration in the force exerted by the resilient means. The disadvantage of this arrangement is that the greater included angle between the downstream conical surface of the valve member and the seating results in an increased volume of fuel trapped in the space between the seating line and the outlet orifice when the valve member is in the closed position. This fuel can dribble through the orifice into the engine and cause unwanted exhaust emissions. If the included angle downstream of the seating line is reduced the aforesaid volume of fuel is reduced but there is an increasing tendency for the effective seating line to move in the downstream direction particularly if the downstream included angle is less than the upstream included angle. As a result the area of the valve member exposed to the fuel pressure in the closed position of the valve member increases and the nozzle opening pressure falls so that there is a deterioration in the performance of the nozzle.
U.S. Pat. No. 4,153,205 also describes how a groove may be formed in the downstream conical surface of the valve member, the groove being spaced in the downstream direction from the seating line. The downstream included angle is less than the upstream included angle and the action of the groove is to minimise the movement as wear takes place, of the seating line in the downstream direction. Thus whilst in use there will be a reduction of the nozzle opening pressure the extent of the reduction will be limited by the action of the groove.
GB 2186632 describes a nozzle arrangement including a nozzle body having an inner conical seating, and a valve needle including a conical surface engageable with the seating, the conical surface of the valve needle being interrupted by an annular groove or slot arranged to alter the flow characteristics of fuel through the nozzle when the valve needle is lifted from the seating. A similar annular groove is provided in the arrangement described in U.S. Pat. No. 1,952,816, in this case the groove being upstream of the effective seating line of the nozzle
The object of the present invention is to provide a fuel injection nozzle of the kind specified in an improved form.